
In chapter 3 the black haired boy hets called into the office of the dad and the dad says that he cant give him special treatment because he is his son so he is told to take a break from work. Later in chapter 8 the light haired one said "were kinda like brothers?" And "were not blood related" when the orange haired one asked if he was dateing the black haired one. when asked in chapter 12 about who taught him how to make the paper arplane the black haired boy said "my dad", we are shown that its the others boys dad who taught him so and when the dad showed up they both said dad (the black haired one thought it and the light haird one said it outloud).
So I'm pretty sure that they share the same dad whether the black haired ones mom remarried to him or if that is his legitimate kid
They're not step brothers yall !